Well Giroud is most definitely plan B for them when the shit hits the fan.
For a classic number 9 big man upfront his technique is superb, not remember Wilshere's goal a few years back? He is brilliant at bringing other players into play (a perfect foil for Griezmann last year at the Euro's) and is good in the air and has a powerful shot on him (remember when he thundered the bar against us from well outside the box?)
He lacks a bit of mobility and pace that Rom has but in the above areas he is far superior and fits into type of player Koeman likes. I mentioned Giroud as one of 2 signings if Rom was to leave last year, would definitely not say no to the guy.
*also I'm sure I read only Harry Kane had a better goals per minute ratio than Giroud this season. I know maybe some of them goals are as a sub against tired defences but still.